Basically, this would implement the ranks 1-30 and A for SeeD operatives only. Your initial rank would come from the quality of your profile (possible ranks 1 through 10), if you started as a SeeD operative. If you started as a candidate, your rank would come from how well you do on the field exam and written exam (1-10 for each, averaged together to get your starting rank). From there, undergoing missions would be the only means of raising your rank.
Each mission would give you a score of 1 through 10; anything over 7 would induce a rank up. This is assuming that you complete the mission parameters.
Finally, it would not be possible to rank down.